In FIG. 15, there is shown one construction example of self-emitting display devices equipped with organic layers (organic electroluminescent device). The display device 1 shown in this drawing is arranged on a transparent substrate 2 made of glass or the like, for example. The display device 1 is composed of an anode 3 arranged on the substrate 2 and made of ITO (indium tin oxide: transparent electrode), an organic layer 4 arranged on the anode 3, and a cathode 5 arranged on the organic layer. The organic layer 4 has, for example, a construction that a hole injection layer 4a, a hole transport layer 4b and an electron-transporting light-emitting layer 4c are stacked together in this order from the side of the anode. With the display device 1 constructed as described above, light produced upon recombination of electrons injected from the cathode and holes injected from the anode within the light-emitting layer 4c is outputted from the side of the substrate 2.
The lifetime of an organic electroluminescent device is generally determined by injected charges, and this problem can be resolved by lowering the initial brightness in each drive. However, the lowering of the initial brightness results in a limitation to the application in its practical use, negates by itself the potential of the organic electroluminescent device, and hence, makes it difficult to realize a next-generation television set.
For the resolution of this problem, it is necessary to increase the brightness without changing the drive current, in other words, to improve the efficiency or to realize a device construction that can obtain a similar brightness even when the drive current is lowered.
Therefore, stacked multiphoton emission devices (MPE devices) with a plurality of organic light-emitting devices arranged one over another have been proposed. Such proposals include the construction of an MPE device (display device 1′) that as illustrated in FIG. 16, plural light-emitting units 4-1, 4-2, . . . , each being formed of an organic layer having at least a light-emitting layer 4c, are arranged one over another via insulating charge generation layers 6, respectively. Each charge generation layer 6 plays a role that upon impression of a voltage, injects holes into the light-emitting unit 4-2 arranged on the cathode 5 side of the charge generation layer 6 and also injects electrons into the light-emitting unit 4-1 arranged on the anode 3 side of the charge generation layer 6, and is composed of a metal oxide such as vanadium oxide (V2O5) or rhenium heptoxide (Re2O7).
To improve the efficiency of the above-described electron injection from the charge generation layer 6 into the light-emitting unit 4-1 on the side of the anode 3, an electron injection layer 7 which acts as an “in-situ reaction inducing layer” may be arranged on the anode 3 side of the charge generation layer 6. As the electron injection layer 7 which acts as such an “in-situ reaction inducing layer”, a mixed layer of bathocuproine (BCP) and metal cesium (Cs) or a stacked film of an (8-quinolinato)lithium complex and aluminum is used, for example.
In a stacked organic electroluminescent device with the light-emitting units 4-1, 4-2, . . . stacked one over another via the charge generation layers 6, respectively, as described above, it is considered possible to double the brightness [cd/A], ideally without any change to the efficiency of light emission [lm/W], when two light-emitting units are stacked together; or to triple the brightness [cd/A], ideally without any change to the efficiency of light emission [lm/W], when three light-emitting units are stacked together (in this connection, see Japanese Patent Laid-Open No. 2003-45676 and Japanese Patent Laid-Open No 2003-272860).
In the display device 1′ of the construction that the light-emitting units 4-1,4-2 are stacked together as described with reference to FIG. 16, however, the material which makes up the electron injection layer 7 as the “in-situ reaction inducting layer” arranged on the anode 3 side of the charge generation layer 6 is very unstable. Therefore, the stoichiometric ratio of the individual materials that makes up the electron injection layer 7 is important, and its imbalance is considered to result in unstableness even as a layer.
For example, BCP is high in complex-forming ability and, if there is a free metal component or an organic material having an active site exists or in a like situation, there is a high possibility that BCP may form a complex with a surrounding material. BCP is, therefore, hardly usable when the stability of a device is taken into consideration. In addition, another problem is considered to exist in that a device making use of BCP has poor reliability in environmental stability.
When the charge generation layer 6 is formed with a metal oxide such as V2O5 or Re2O7 in such a stacked organic electroluminescent device, the efficiency of electron injection by direct contact of a general electron transport layer of Alq3 or the like with the charge generation layer 6 is extremely low. Accordingly, the formation of an interface on the anode 3 side of the charge generation layer 6 becomes an extremely important point.
An object of the present invention is, therefore, to provide a stacked display device having light-emitting units composed of organic layers, respectively, and stacked together, which is provided with improved environmental stability by the use of a stable material, is provided with an improved efficiency of charge injection from a charge generation layer, which are held between each two adjacent ones of the light-emitting units, into the two light-emitting units, is hence high in brightness and excellent in long-term reliability, and is easy to fabricate.